1. Field of the Invention
This invention relates to in-line exhaust system for an v-configured, transverse-mounted internal combustion engine having a front manifold having a first exhaust stream and a rear manifold rear having a second exhaust stream.
2. Description of the Prior Art
At the present time, automotive exhaust systems equipped with catalytic conversion systems generally comprise an exhaust line connecting with a converter housing, the housing enclosing a ceramic or metal honeycomb substrate supporting an oxidation or, more commonly, a three-way emissions control catalyst. The three-way catalyst operates to oxidize carbon monoxide and unburned hydrocarbons present in the exhaust stream and, with proper control of exhaust stream stoichiometry, to at least partially reduce higher oxides of nitrogen (NOx) contained therein.
Tightened emissions standards for automotive gasoline engines have placed higher demands on the performance of these catalytic conversion systems. Particularly critical for overall system performance is performance during the so-called "cold start" phase of engine operation. This is the period of engine operation covering approximately the first 60 seconds after cold engine start and prior to startup or "light off" of the catalytic converter, during which the highest concentrations of unburned hydrocarbons are released into the atmosphere.
One type of exhaust system designed specifically to address the cold-start problem provides a hydrocarbon adsorber in the exhaust line. The adsorber operates to trap unburned hydrocarbons emitted during engine startup, and then to release those hydrocarbons to a catalytic converter after converter light-off has been achieved. A preferred configuration for an adsorber in such systems is a honeycomb structure, similar in construction to a catalyst support honeycomb but composed of, or supporting, a coating of a hydrocarbon adsorbent such as carbon, zeolite, or another adsorber material.
Examples of recently developed cold-start engine emissions control systems of this type are disclosed in published patent applications WO 95/18292, EP 0661098 and EP 0697505 (Hertl et al). Two further examples of such system are described in copending, commonly assigned U.S. patent application Ser. No. 08/578,003 (Brown et al.) filed Dec. 22, 1995 and entitled "Exhaust System with a Negative Flow Fluidics Apparatus" and U.S. patent application Ser. No. 08/685,130 of J. Anderson et al. filed Jul. 24, 1996 and entitled "Exhaust Gas Fluidics Apparatus".
A common feature of "in-line" systems is a ported honeycomb adsorber, i.e., an adsorber comprising a by-pass port integral with its structure, located downstream of a main or light-off catalytic converter but positioned upstream of a second or so-called "burn-off" catalytic converter. This adsorber functions to trap the hydrocarbons released at engine startup and slowly desorb and release the hydrocarbons to the burn-off converter as the adsorber is heated by the warming exhaust gases. A particular advantage of the ported adsorber design is the faster light-off of the burn-off converter due to exposure of that converter to the hot exhaust gases passing directly through the adsorber port. In the design of Hertl et al., Brown et. al. and Anderson et al., control over the flow of the exhaust gases through or past the adsorber is secured by means of a fluidic diverter which delivers a control gas stream for diverting the exhaust gases toward or away from the adsorber port in the course of engine operation.
V-engines mounted in the transverse position generate two exhaust gases which exhibit two different temperatures when combined together at some point prior to entering a main exhaust pipe. The exhaust coming from the front manifold is cooled as a result of heat loss along the length of a crossover pipe which runs from the front of the engine towards the back and is generally 3 to 4 feet in length. The exhaust from the rear manifold travels a much lesser distance, generally less than a foot, prior reaching a junction where it is mixed with the exhaust from the front manifold. As such, the exhaust gas from the rear manifold exhibits a much higher temperature at the point where the gases are mixed. At this point the temperatures typically differ by as much as 150.degree. C. and as a result, the overall temperature of the mixed exhaust stream is greatly reduced when compared to the single stream of the aforementioned systems described above. Incorporation of the aforementioned in-line exhaust systems at a position downstream of the junction would therefore result in a inefficient configuration which would exhibit a delayed light-off of the main catalyst, when compared to the aforementioned single stream systems. As such, incorporation of these in-line exhaust systems into v-engines, as described would result in an exhaust system which would exhibit poor overall exhaust purification performance.